Method of galvanizing ferrous metal strip



1959 J. L. GREENE ETAL 2,894,850 METHOD OF GALVANIZING'FERROUS METAL STRIP fiooooo k y oooooob United States Patent Birmingham, Mich, assignors to General Motors Corporation, Detroit, Mich., a corporation of Delaware Application May '14, 1958, Serial No. 735,316 7 Claims. (Cl. 117-55) This invention relates to a method of coating a base metal with another molten metal and more particularly to a method of galvanizing ferrous metal strip or sheet material on one side only.

Galvanized sheet metal is conventionally used in applications where the metal must necessarily be exposed to the corrosive effects of the elements and the like. It has been considered for use in automobile body construction to improve corrosion resistance, particularly on the interior surfaces of the body where rusting is generally most severe. However, commercial galvanized sheet, which is zinc coated on both sides, has never been generally accepted for automobile body construction on a commercial basis because of complications arising from the adherence of zinc to the electrode tips in the process of resistance welding the galvanized parts together. Moreover, the surface quality of galvanized sheet for exterior body surfaces is generally inferior to bare sheet steel.

For these reasons it is desirable to construct critical body components of galvanized sheet steel which is zinc coated on one side only, wherein the zinc coated surface is provided on the interior surfaces. This construction provides protective qualities on the interior surfaces where it is most needed and leaves the exterior surfaces uncoated for maximum weldability and surface finishing. These exterior uncoated surfaces are ultimately protected by a conventional painting treatment.

It is the basic object of this invention toprovide a method for galvanizing ferrous metal sheet or strip material on one side thereof only and leave the opposite surface free of the zinc coating and in a clean condition for optimum weldability and surface finishing.

It is a further object of this invention to provide a method of galvanizing sheet ferrous material on one side only which may be utilized in connection with galvanizing processes currently in commercial use. These and other objects are accomplished by a process including the steps of first cleaning or degreasing the sheet material by any well-known manner; coating one side of the sheet with an alkali metal aluminate solution, preferably containing small quantities of a suitable nonionic wetting agent which will not dissociate in a strongly alkaline solution; drying the alkali metal aluminate coated sheet; subjecting the dried sheet to a reducing atmosphere at temperatures ranging from about 1250 F. to l'450 F.; then dipping the sheet into molten zinc while still protected by the reducing atmosphere; and, finally, removing the sheet from the molten zinc, cooling it and removing the alkali metal aluminate coating by brushing or other suitable means.

Other objects and advantages of this invention will more fully appear from the following detailed description of a preferred embodiment of the invention, reference being made to the accompanying drawing which shows a schematic view of one form of apparatus suitable for carrying out the invention.

Incarrying out the basic aspects of the invention a steel sheet is first cleaned by dipping it in a cleaning bath containing a suitable detergent. Any well-known degreasingor cleaning method, such as by means of an electrolytic caustic solution or vapor degreasing, may be used. After the sheet has been suitably cleaned, one side thereof is coated with an aqueous alkali metal aluminate solution, preferably containing small quantities of a suitable wetting agent. An alkali metal aluminate coating of a thickness of .001 to .002 inch has been found satisfactory. The solution may be suitably applied by brushing. However, other methods of application, such as spraying or rolling may also be used. A preferred solution for this purpose is an aqueous solution containing about 20% sodium aluminate (NaAlO and about 0.3%

'of a wetting agent in the form of polyoxyethylene sorbitan monolaurate.

Although a 20% sodium aluminate solution represents a preferred concentration, sodium aluminate solutions as low as 5% may be used. Solutions below 5% are undesirable, since below this concentration the sodium aluminate tends to dissociate to sodium hydroxide and precipitated aluminum oxide with a result that these materials would be undesirably applied to the steel sheet. Higher concentrations of the sodium aluminate are also suitable, such higher concentrations being limited by the saturation characteristics of the sodium aluminate. Other alkali metal aluminates may also be used in place of the sodium aluminate as well as mixtures of alkali metal aluminates. Sodium and potassium aluminates are preferred because of their availability and high water solubility. Lithium aluminate is less desirable because of its low solubility in water while other alkali metal aluminates, such as rubidium and cesium aluminate are of limited use, as a practical matter, due to the scarcity and relatively high cost of these alkali metals.

In general it has been found that any of the wetting agents of the non-ionic type which are not subject to dissociation in strong alkaline solutions are suitable. Specific examples of wetting agents which have been successfully used include sorbitan mohooleate and mixtures of polyethylene oxide and polypropylene oxide. The Wetting agent is preferably presentin small but effective amounts, such as will permit an ethcient application of the alkali metal aluminate solution to the cleaned sheet metal.

After the alkali metal aluminate solution is applied to one side of the steel sheet, the sheet is preferably subjected to heat at temperatures ranging from 250 F. to 300 F. for a time sufficient to cause the sodium aluminate coating to dry. It has been found that drying temperatures up to 700 F. may also be used in the drying step whereby a very rapid drying may be accomplished. Experimental tests have indicated that the sodium aluminate coating, dried and baked at 600 F. to 700 F., involves an oxidation of the steel beneath the sodium aluminate coating but that this oxide is reduced so as to' provide a clean steel sheet in a subsequent reduction process, as will be hereinafter described.

After the sodium aluminate coating has been dried as described, the sheet is then placed in a reducing atmosphere heated to about 1400 F. for about 5 minutes or a time sufficient to remove all oxides from the metal. Reducing temperatures ranging from 1250 F. to 1450 F. may be satisfactorily used with an appropriate adjust ment in time. A reducing atmosphere of hydrogen having a dew point of F. has been found highly satisfactory for the purpose. Another suitable reducing atmosphere includes typically 12.5% hydrogen, 10.5% carbon monoxide, 5.0% carbon dioxide, 0.5% methane, 0.8% water, and 70.7% nitrogen. Another suitable re ducing atmosphere which may be used is cracked ammonia having a dew point of about 80 F.

After the sheet metal has been suitably exposed to the reducing atmosphere so as to fully remove any oxidized surface thereon and While still under the protective reducing atmosphere, it is dipped for about 5 to 30 seconds into molten zinc maintained at about 850 F. 'After the dipping step the sheet is permitted to cool and the coating to solidify by guiding it a sufficient distance in the atmosphere. The sodium aluminate coating is removed by a simple water rinse and brushing operation. It is to be understood that an important aspect of this invention resides in the use of the alkali metal aluminate solution which will remain adhered to the metal sheet during the galvanizing process, prevent the adherence of the zinc to the aluminate coated portions of the sheet metal and be readily removed by a simple operation such as brushing so as to provide a clean ferrous metal surface which is readily amendable to Welding operations and the reception of surface coating such as paint and the like. No other material has been found which will prevent the adherence of zinc and yet be readily removed after being subjected to the reducing heat treatment of 1250 F. or more.

The process as described above, as is readily apparent, may be conducted on a batch basis. However, the process may be readily adaptable to operation on a continuous basis as is illustrated in the drawing. Sheet -metal or strip is provided in the form of a roll mounted on a suitable rotatable supporting mechanism 12. In accordance with the process of the invention, the sheet is guided by suitable rollers 14, 16, 18 and 20, and continuously passed into and out of a cleaning bath 22 contained in tank 24. The tank 24 is, of course, suffi- -ciently large to permit the sheet metal to be adequately cleaned as it passes through. Thereafter the sheet is passed over a roller 26 which is partially submerged in a sodium aluminate solution 28 contained in a tank 30, whereby the underside of the sheet is coated with the sodium aluminate solution. Thereafter the sheet is passed through an electrically heated drying oven 32 maintained preferably at about 250 F. to 300 F. The heating furnace is, of course, of sufficient length and the temperature is sutficiently high so that the sodium aluminate coating will dry as it passes therethrough.

After leaving the heating oven 32, the sheet supported by rollers 37 is passed into an electrically heated furnace 34 containing a reducing atmosphere in its chamber 36. A suitable sealing means 38, such as sealing rollers or an asbestos sheet, is provided in the furnace opening to retain the reducing atmosphere therein. The furnace '34 maintains the reducing atmosphere at a temperature preferably ranging from 1250 F. to 1450 F. After leaving the reducing furnace 34, the strip, guided by rollers 40 and 4-2, is dipped into molten zinc 46 contained in a cuucible or suitable container 48. The sheet is then guided downwardly by means of rollers 50 and 52, and thence into a tank 54 containing a water rinse 56 containing a suitable rust inhibitor such as sodium dichromate. Within the Water rinse, the sheet is passed beneath a roller 58 and over rotary bristle brush 60 positioned adjacent the sodium aluminate coated surface of the sheet and rotating in a clock-wise direction. The brushing action together with the water rinse is effective in removing the aluminate coating from the sheet so as to provide a clean, zinc-free surface. Finally, the sheet is passed through a drying oven 62 which includes hot air blast nozzles 64 for the purpose of drying the sheet. Finally, the sheet is coiled on a roller 66.

It is to be understood that the various components of the apparatus for elfecting the continuous method described are of sufiicient length and of suitable design to effect the various steps in the process previously outlined. The various guide roller mechanisms, including the rollers 37, 40 and 42 are preferably of suitable design so as to grip the sheet at the edges thereof so as not to remove or destroy significant amounts of the alkali metal aluminate coating from the sheet as .it passes from the alkali metal aluminate container 28 and through the zinc dip bath 46. In general, it is only necessary to apply power to the roller 66 to draw the sheet 10 through the apparatus. However, power may be applied to other rollers to efficiently move the sheet through the apparatus.

Although the invention has been described in terms of galvanizing using a zinc bath, it is to be understood that the invention comprehends the addition of small amounts of aluminum to the molten zinc for the purpose of suppressing iron-zinc compound formations and the addition of small amounts of other metals such as lead, antimony, cadmium and tin used either singly or in various combinations for the purpose of improving the quality of the zinc coating.

While this invention has been described with reference to certain preferred embodiments and conditions, it will be understood that the invention is not limited thereby and numerous changes and modifications will be apparent to those skilled in the art without departing from the spirit and principles of the invention.

We claim:

1. A process for galvanizing a portion only of a ferrous metal article, the steps comprising applying an alkali metal aluminate solution to the portion of the article which is not to be galvanized, drying said coating, surrounding said article by a reducing atmosphere and subjecting said article to heat in a temperature range of from about 1250 F. to 1450 F. while surrounded by said reducing atmosphere for a time suflicient to reduce the oxides on said ferrous metal, and dipping said article in a galvanizing bath while still under said reducing atmosphere.

2. A process for galvanizing a portion only of the ferrous metal article, the steps comprising applying an alkali metal aluminate solution containing a small but effective amount of a wetting agent to the portion of the article which is not to be galvanized, drying said coating by subjecting said article to heat in a temperature range of from 250 F. to 700 F., surrounding said article by a reducing atmosphere and subjecting said article to heat in a temperature range of from about 1250 F. to 1450 F. while surrounded by said reducing atmosphere for a time sufiicient to reduce the oxides on said ferrous metal, and dipping said article in a galvanizing bath while still under said reducing atmosphere.

3. A process for galvanizing a portion only of a ferrous metal article, the steps comprising cleaning said article, applying a sodium aluminate solution containing a small but effective amount of a non-ionic wetting agent to the portion of the article which is not to be galvanized, drying said coating by subjecting the same to heat at a temperature range from 250 F. to 700 F., surrounding said article by a reducing atmosphere and subjecting said article to heat at a temperature range of from about 1250 F. to 1450 F. While in said reducing atmosphere for a time sufficient to reduce the oxides on said ferrous metal, dipping said article in a galvanizing bath while under said reducing atmosphere,

and removing the soditun aluminate solution.

4. A process for galvanizing one side only of a sheet metal article, the steps comprising applying an aqueous alkali aluminate solution containing a small but effective amount of a wetting agent to the side of said sheet which is not to be galvanized, drying said coating by subjecting said sheet to heat in a temperature range from 250 F. to 700 F., surrounding said sheet by a reducing atmosphere and subjecting said sheet to heat in a temperature range of from about 1250 F. to 1450 F. while surrounded by said reducing atmosphere for a time sufiicient to reduce the oxides on said sheet, dipping said article in a galvanizing bath while still under said reducing atmosphere and thereafter removing said alkali metal aluminate coating.

5. A process for galvanizing a ferrous metal article, the steps comprising applying an aqueous solution containing from about 5 to 20% of an alkali metal aluminate and a small but effective amount of a Wetting agent to the portion of said article which is not to be galvanized, drying said coating by subjecting said article to heat in a temperature range of from 250 F. to 300 F., surrounding said article by a reducing atmosphere and subjecting the article to heat in a temperature range of from about 1250 F. to 1450 F. while surrounded by said reducing atmosphere for a time sufiicient to reduce the oxides on said ferrous metal, dipping said article in a galvanizing bath While still under said reducing atmosphere and removing alkali metal silicate coating by means of a brush and a Water rinse.

6. A process for galvanizing one side only of a sheet steel article, the steps comprising cleaning said article, applying an aqueous solution containing about 20% of sodium aluminate and about 0.3% of a non-ionic Wetting agent, drying said coating by subjecting said article to heat in a temperature range of from about 250 F. to 300 F surrounding said article in a reducing atmosphere and heating the article within a temperature range of about 1250 F. to 1450 F. while slnrounded by said atmosphere for a time sufficient to reduce the oxides on said steel article, dipping said article in a galvanizing bath while under said reducing atmosphere and removing said sodium aluminate coating by brushing and a water rinse.

7. A process for galvanizing a portion only of a ferrous metal article, the steps comprising applying an alkali metal aluminate solution to the portion of the article which is not to be galvanized, drying said coating, surrounding said article by a reducing atmosphere and subjecting said article to heat at a temperature of at least 1250 F. while surrounded by said reducing atmosphere for a time sufficient to reduce the oxides on said ferrous metal, and dipping said article in a galvanizing bath while still under said reducing atmosphere.

No references cited. 

1. A PROCESS FOR GALVANIZING A PORTION ONLY OF A FERROUS MEWTAL ARTICLE, THE STEPS COMPRISING APPLYING AN ALKALI METAL ALUMINATE SOLUTION TO THE PORTION OF THE ARTICLE WHICH IS NOT TO BE GALVANIZED, DRYING SAID COATING, SURROUNDING SAID ARTICLE BY A REDUCING ATMOSPHERE AND SUBJECTING SAID ARTICLE TO HEAT IN A TEMPERATURE RANGE OF FROM ABOUT 1250* F. TO 1450* F. WHILE SURROUNDED BY SAID REDUCING ATMOSPHERE FOR A TIME SUFFICIENT TO REDUCE THE OXIDES ON SAID FERROUS METAL, AND DIPPING SAID ARTICLE IN A GALVANIZIONG BATH WHILE STILL UNDER SAID REDUCING ATMOSPHERE. 